1. Field of the Invention
This invention relates generally to a method and apparatus for rapid thermal heating of a semiconductor wafer and, in particular, to a heating system which heats the wafer through its edge.
2. Description of Related Art
In processing semiconductor wafers used to manufacture integrated circuits (IC's) and other electronic devices, wafers are typically introduced into the processing tool at ambient temperature that is at or near room temperature. On the other hand, many process steps are carried out at elevated temperatures. Such processes include thermal oxidation or nitridation of surfaces, annealing of wafers to remove water vapor (i.e. degassing), chemical vapor deposition (CVD) of thin films, and the like. It is therefore often necessary to raise a wafer from a relatively low temperature to an elevated temperature. In order to keep wafer throughput high and cost-of-ownership low, the time to heat the wafer to its process temperature should be kept as short as possible. In addition, since reaction rates are often highly dependent on temperature (film deposition rate, water outgassing rate, and the like), it is also desired to achieve a uniform temperature across the wafer so that processing of the wafer is likewise uniform across its entire surface.
To address the need for rapid thermal processing (RTP), the IC industry has widely adopted radiant energy sources consisting of high-intensity lamps housed in the upper and/or lower surface of the processing chamber. Examples of such rapid thermal processing systems are those shown in U.S. Pat. Nos. 6,072,160 and 6,122,440. In these and other similar rapid thermal processing systems, zone-based radiant heating is utilized through the use of multiple spot lamps, with each generally heating different areas of one or both of the wafer surfaces.
Such heating lamps often use as a light source a tungsten filament lamp in a quartz envelope, typically including a halogen gas to improve lifetime. Blackbody radiation emitted from the hot filament (typically at temperature 2000–3000° K) provides the illumination. These quartz-tungsten or quartz-halogen lamps are inexpensive and widely deployed to heat silicon wafers. The visible radiation from such lamps is strongly absorbed by the silicon, which has an optical absorption coefficient α greater than 104-cm1 for visible light (wavelength ˜380–780 nm). However, most of the radiation produced by these hot filament, black body sources has energy in the near-infrared region. For example, ˜70% of the radiation from a tungsten filament at 3000° K lies in the infrared. The energy of these infrared photons is less than the 1.1 eV bandgap of silicon, and as a result they pass through the silicon wafer (which typically has a thickness less than 0.1 cm) with little or no absorption when the wafer is at room temperature (absorption coefficient α less than 1 cm−1). At elevated temperatures above about 400° C., where free carrier absorption by electrons is strong in silicon, the thin wafer strongly absorbs both visible and near infrared radiation. Therefore, once the wafer gets hot enough, there is strong optical coupling to the wafer and rapid heating with tungsten-filament lamps becomes far more effective. However, the problem remains how to rapidly heat a silicon wafer with a conventional quartz halogen lamp when the wafer is initially at or near room temperature. In addition, the IC industry continues to drive process temperatures lower such that many processes are now carried out close to or below the temperature where strong optical coupling can be exploited for rapid heating with quartz-halogen lamps.
The lamp heating of wafers is typically done with broad area illumination from the wafer front and/or back side, in which the planar surfaces of the wafer are exposed to the light. While the area of illumination is large, for example, 720 cm2 for a 300 mm diameter wafer, the wafer thickness is very small, for example, 0.075 cm. This thickness is much less than the absorption depth z of the radiation at room temperature (z=1/α>1 cm) and, as a result, the percentage of incident light absorbed is very small.